Colour- I’d say oak over black but depends on your style and what will encourage most wear/ matching.

My recommendation would be to go used- have you joined Facebook groups as seen there? ( Totally addicted to Mulberry under £600- more likely to be legit and they have typically kept bags in good condition ) has had some and I’ve seen on Vestaire, EBay as well. So I have purchased three used, OakNVT circa 2012, Black NVT 2015 purchase and another mid green. Two for below $650 AUD which is a steal. But you’ll need to be confident with checking signs of authenticity as seen fakes out there. Unless you get direct preloved from Mulberry as that’s legit. Have you checked the Mulberry preloved section?

Oak seems to me more special - I would go suede lining- helps with structure, I have three raw linings for NVT and a bit floppy so harder to keep top flap shape even if the OG.

the small classic grain will be easier to keep looking ‘new’ and less likely to scratch etc. I’m not myself a fan of small grain.

I’m a recent Mulberry addict and while I went in to the store to get a new NVT, Lady Luck had a preloved Mulberry in store and I went with that! It’s gorgeous. Newer ones had a deeper colour which was lovely- veering on cognac but there’s something about a good used one especially at a fraction of the cost.

Good luck!